1. Field of the Invention
The present invention relates to a neutron shielding material and more particularly to a neutron shielding material which is preferably applicable to radiation shielding parts such as reactor vessels, radioactive material treating facilities such as a nuclear fuel reprocessing facility, a spent fuel storing facility, and an accelerator facility, a cask for transporting radioactive materials, and a cask for storing radioactive materials.
2. Description of the Prior Art
Spent fuel assembly are taken out from an atomic reactor, stored in water-cooled pools at the atomic power plant site for a preset time period to attenuate radiation dose and calorific power, and then transported to a processing facility such as a fuel reprocessing factory and the like. Recently in countries outside Japan, the spent nuclear fuel assembly are transported to a centralized storage facility (dry storage facility) and stored there. A radioactive storing shell called a metal cask is used to carry the spent nuclear fuel assembly from the atomic power plant site to such a facility and store there.
A metal cask consists of an outer shell which forms the container, an inner shell having heat-transferable fins made of high heat-conductivity metal plates such as copper or aluminum spaced on the outer periphery of the inner shell, and a metallic basket placed inside the inner shell. The space between the outer and inner shells is filled with a hardened resin which works as a neutron shielding material. The inner shell having an opening on the top is made of carbon steel and can shield gamma rays. The metallic basket has a plurality of cells each of which is designed to store a spent fuel aggregate. One metallic basket can store a total of 30 to 70 spent fuel assembly. The opening of the inner shell is closed with a primary lid to prevent leakage of radioactive materials and a secondary lid which is placed over the primary lid.
The resin working as a neutron shielding material is a material containing a lot of hydrogen atoms, that is a material having a high hydrogen number density. Among various kinds of high polymer compounds, the metal casks usually employ epoxy resins because the relationship of heat resistance and hydrogen number density is well balanced. In this case, the resin is a homogeneous mixture of base liquid epoxy resin, amine type hardener, aluminum hydroxide which gives flame resistance to the resin, and boron carbide which works as a neutron absorbing material. This liquid resin is poured into the space surrounded by the inner shell, the outer shell and the heat-transferable fins and hardened there at room temperature.
Below will be explained neutron shielding materials using thermo-setting resins such as epoxy resin which are applied to other than metal casks. Japanese Application Patent Laid-open Publications No. 06-148388 discloses a neutron shielding material which is obtained by mixing a multifunctional epoxy resin, a poly-amine mixture, and an imidazole compound, and reacting thereof to harden at room temperature. Japanese Application Patent Laid-open Publications No. Hei 06-180388 discloses a neutron shielding material which is hardened under pressure and heating with a phenol resin as a binder.
It has been discussed whether dry storage of spent fuel in and outside the nuclear power plant site is available for loose storage of spent fuel assembly in the water-cooled pools. In future, dry storage will be available for spent fuel assembly which are not stored so long in the water-cooled pools and further for high-burnup fuel assembly (45 GWd/ton). Such fuel assembly have great calorific power due to decays of fission-produced nuclides and transuranic elements. When the number of spent fuel assembly to be stored in such a metal cask increases, the neutron shielding material will have a greater thermal load as its thermal conductivity is smaller than that of metals.
It is an object of the present invention to provide a neutron shielding material which can be available at a higher temperature.
The above object of the present invention can be attained by constructing a neutron shielding material with the hardened material prepared by mixing a base resin which contains a compound including two or more epoxy groups in the molecule as at least one component with a hardener for opening said epoxy rings and polymerizing thereof at a temperature higher than room temperature.
Even when the neutron shielding material of the present invention is kept at 150xc2x0 C. to 200xc2x0 C., its neutron shielding performance will not go down as the hydrogen number density of the neutron shielding material has a very little rate of reduction at such a high temperature. The spent fuel storing shell employing the neutron shielding material of the present invention can store more spent fuel assembly which are stored for a short period in a water-cooled pool or high-burnup fuel assembly.
Another object of the present invention can be attained by constructing a neutron shielding material with the hardened material prepared by mixing a base resin which is prepared by mixing a base resin which contains a compound including two or more epoxy groups in the molecule as at least one component with a hardener for opening said epoxy rings and polymerizing thereof, wherein the setting temperature is higher than room temperature.
We inventors cleared up the problems involved in storing spent fuel assembly which are stored for a short period in a water-cooled pool or high-burnup fuel assembly in metal casks and discussed measures to solve the problems. The result of the discussion will be explained in detail below.
The spent fuel assembly which are stored for a short period in a water-cooled pool or high-burnup spent fuel assembly generate high calorific power due to decays of fission products and transuranic elements. We found xe2x80x9cthe temperature of the neutron shielding material in the metal cask goes up to 150xc2x0 C. to 200xc2x0 C. when a lot of assembly are stored in a single metal cask.xe2x80x9d
When heated up, the neutron shielding material having a high-polymer compound as its main component becomes oxidized and deteriorated by heat and oxygen or gradually decomposed by radioactive rays such as gamma rays and neutrons and loses hydrogen atoms. As the result, the neutron shielding material gradually loses its neutron shielding performance. The rate of losing hydrogen atoms goes greater as the temperature becomes higher. For long-term storage of spent fuel assembly which are stored for a short period in a water-cooled pool and high-burnup spent fuel assembly (which are also called high exothermic spent fuel assembly) at a high temperature densely in a metal cask, a neutron shielding material must be developed which loses hydrogen atoms so slowly and does not lose the neutron shielding performance for a preset time period at high temperature. It is possible to suppress the radiation dose low on the surface of the metal cask if the hydrogen atom losing rate of the neutron shielding material is below the decaying rate of the neutron-emitting nuclides in the spent fuel assembly. Judging from these, one of measures in densely storing high exothermic spent fuel assembly is to produce neutron shielding material by using a high-polymer compound which has a high hydrogen number density and is reluctant to lose hydrogen atoms under high-temperature conditions.
We inventors have made various discussions, studies and researches to realize neutron shielding materials which are slow to lose hydrogen atoms at 150xc2x0 C. to 200xc2x0 C. and developed by mainly using epoxy resins because the epoxy resins have good heat resistance, neutron-shielding performance, and dimensional stability of molded products. The term xe2x80x9cepoxy resinxe2x80x9d here mainly means so-called 2-component hardening type epoxy resin. The 2-component hardening type epoxy resin comprises a base epoxy resin having two or more epoxy groups in the molecule and a hardening agent which is used together to harden. The epoxy resins are classified into three (room-temperature setting type, medium-temperature setting type, and high-temperature setting type) according to setting conditions. Medium- and high-temperature setting types are sometime called heat-setting epoxy resins. This classification is dependent upon combinations of base resin and hardener. As a rough example, a bisphenol A type epoxy resin is hardened by an aliphatic polyamine type hardener at room temperature. Similarly, the bisphenol A type epoxy resin is hardened by alicyclic polyamine and polyamide amine type hardeners respectively at room and medium temperatures. To harden the bisphenol A type epoxy resins at a high temperature, aromatic polyamine type hardeners and acid anhydride are used. The medium-temperature setting type epoxy resins are the epoxy resins whose primary setting temperature is loosely in 40xc2x0 C. to 80xc2x0 C. and the high-temperature setting type epoxy resins are the epoxy resins whose primary setting temperature is 80xc2x0 C. or higher.
In general, it is well known that the hardened resin product can have higher heat resistance as the setting temperature goes higher. In other words, the high-temperature setting type resins are more heat resistant than the medium-temperature setting type resins when they are hardened. Similarly, the medium-temperature setting type resins are more heat resistant than the room-temperature setting type resins. The xe2x80x9cheat resistancexe2x80x9d here is used to determine a high temperature limit allowable for the resin in respect to the mechanical strength, using a glass-transition temperature or a heat distortion temperature as the index. Contrarily, the thermal resistance index specific to neutron shielding materials is not such an index that is related to the mechanical strength, but related to the rate at which the hydrogen number density reduces and approximately to the rate at which the weight reduces by heat. We inventors hardened base epoxy resins under various conditions and evaluated the rates of resins (heat resistance) at which the weight reduces by heat, considering that the resins are applicable to neutron shielding materials. As the result, we discovered that the resins which are hardened at higher temperature have slower weight reduction rates.
As shown in FIG. 1, the hardened resin prepared by hardening bisphenol A type epoxy resin with an aromatic amine or acid anhydride at a high temperature has slower weight reduction rates (at 200xc2x0 C.) than the hardened resin prepared by hardening bisphenol A type epoxy resin with an aliphatic polyamine or polyamideamine hardener at room temperature. Similarly, the hardened resin prepared by hardening bisphenol A type epoxy resin with an alicyclic amine at a high temperature explicitly has slower weight reduction rates than the hardened resin prepared by hardening bisphenol A type epoxy resin with an alicyclic amine at room temperature. Further, we hardened non-bisphenol A epoxy resins (e.g. bisphenol F, phenol novolac, and glycidyl amine type epoxy resins) with acid anhydride at a high temperature and found that their weight reduction rates due to heat could be extremely smaller as shown in FIG. 2. Judging from these test results, we inventors have concluded that medium- and high-temperature setting type epoxy resins can be applied as neutron shielding materials to be used in high-temperature environments.
As the analytic result of elementary compositions of said hardened epoxy resins, we found xe2x80x9cthe hydrogen number densities of the hardened epoxy resins in combination with acid anhydride or aromatic amine at high temperature are usually lower than those of the epoxy resins hardened at room temperature.xe2x80x9d Also in such a case, the neutron shielding material comprising the above hardened epoxy resin can have a required neutron shielding performance by increasing its thickness. However, we have found that we need not increase the thickness of the neutron shielding material (or increase the thickness just a little) if we take some measures to increase its hydrogen number density. The first measure to increase the hydrogen number density of epoxy resins without destroying the high heat resistance is substituting the bisphenol A base epoxy resin by alicyclic glycidyl ether or the like which contains more hydrogen atoms and hardening thereof at a high temperature. To increase the hydrogen number density of the hardened epoxy resins, the second measure is to add metal hydride such as titanium hydride when the epoxy resins are in combination with acid anhydride or aromatic amine as a hardener. In case alicyclic amine is used as a hardener, the hydrogen number density of the hardened epoxy resin can be increased much more when metallic hydride is added. The third measure is to substitute part of acid anhydride (when the acid anhydride is used as a hardener) by an amine type hardener whose quantity to the base resin can be less. This increases the rate of amine type hardener in the base resin having a high hydrogen content and consequently increases the hydrogen number density of the hardened resin. The present invention contains any of the above measures to increase the hydrogen number densities of the hardened epoxy resins. The heat-setting epoxy resins scarcely lose the neutron shielding performance for a long time period as they are high heat resistant and slow to lose hydrogen atoms. Particularly, the first, second, or third measure can suppress the thickness of the neutron shielding material whose hydrogen number density is low.
Judging from the empirical rule that the heat-setting type epoxy resins which are superior in heat resistance have a lower hydrogen number density, it had been assumed that the measure of using alicyclic glycidyl ether as the base resin might reduce the heat resistance of the resin. However, our test results have told that, when the normal bisphenol A epoxy resin is substituted partially or wholly by alicyclic di-glycidyl ether, the weight reducing rate of the hardened resin at 200xc2x0 C. is extremely low if the resin is heat-hardened by acid anhydride. From this test result, we inventors hit on an idea that a high heat-resistant neutron shielding material having an excellent neutron shielding performance can be produced from alicyclic di-glycidyl ether. It has been well-known that adding metal hydride to room-temperature type epoxy resin for neutron shielding increase the hydrogen number density effectively. However, when metal hydride is added to a heat-setting type epoxy resin (not a room-temperature setting type epoxy resin) to be used as a neutron shielding material, a peculiar problem occurs. Under a heated setting condition, metal hydride and a hardener such as acid anhydride react very quickly and the reactant may lose hydrogen atoms easily. As for this problem, we inventors added metal hydride to the mixture of heat-setting type epoxy resin and a hardener to be hardened and evaluated the reaction. As the result, we found xe2x80x9cmetal hydride reacts with neither hardener nor base resin.xe2x80x9d Judging from this test result, we hit on an idea that we can increase the hydrogen number density of the heat-setting type epoxy resin by adding metal hydride to the epoxy resin.
It sometimes happens that hydrate of metal oxide is added as a fire retardant to the epoxy resin to give flame resistance to the epoxy resin to be used as neutron shielding material. For example, a neutron shielding material used in a metal cask is an example of neutron shielding material including a fire retardant. Usually, hydrated aluminum usually known as alumina trihydrate is added to the room-temperature setting type epoxy resin. When heat-setting epoxy resins are applied as neutron shielding materials, it is peculiar that water must be removed from the fire retardant while the epoxy resin is heated and hardened. However, this dehydration of the fire retardant will reduce the hydrogen number density of the neutron shielding material and consequently this leads to a reduction in the neutron shielding performance of the epoxy resin. The epoxy resin is sometimes heated up to about 200xc2x0 C. while setting is in progress because the neutron shielding material of the present invention is assumed to be used mainly under a temperature condition of 150xc2x0 C. to 200xc2x0 C. In other words, any fire retardant that starts to dehydrate below 200xc2x0 C. may not be available to the heat-setting type epoxy resins. Therefore, we must clarify a standard for selecting fire retardants.
In general, it has been known that alumina trihydrate starts dehydration at about 200xc2x0 C. This temperature is evaluated by the differential thermal calorimetry and thermogravimetric analysis that heats up comparatively slowly at a rate of a few degrees per minute for measurement. However, after our more careful and exact differential thermal calorimetry and thermogravimetric analysis, we found that alumina trihydrate already started dehydration at about 170xc2x0 C. at a significant rate. Therefore, in some cases, alumina trihydrate cannot be applied to the heat-setting epoxy resins. Similarly, it has been known that magnesium hydroxide starts dehydration at about 310xc2x0 C. However, after our more careful and exact analysis, we found that magnesium hydroxide started dehydration at about 290xc2x0 C. at a significant rate. This dehydration starting temperature is fully higher than the maximum hardening temperature (about 290xc2x0 C.) of heat-setting epoxy resins and magnesium hydroxide can scarcely dehydrate at actual hardening temperature. To ascertain this, we inventors kept magnesium hydroxide at 200xc2x0 C. for 200 hours and measured its weight reduction rate. The weight reduction rate was 0.1% or less. Judging from the above, any of alumina trihydrate and magnesium hydroxide can be added as a fire retardant to a heat-setting type epoxy resin for neutron shielding which is hardened at medium temperature in combination with an alicyclic polyamine or polyamide amine hardener. When the epoxy resin is hardened by an aromatic amine or acid anhydride hardener at a maximum hardening temperature (about 200xc2x0 C.), a preferable fire retardant is magnesium hydroxide. This is our explicit standard for selecting fire retardants. By the way, magnesium hydroxide dehydrates vigorously at about 350xc2x0 C. Some room-temperature setting epoxy resins thermally decompose at lower temperatures. However, magnesium hydroxide cannot function as a fire retardant for such epoxy resins if the epoxy resins may be heated up to 300xc2x0 C. Contrarily, almost all heat-setting type epoxy resins thermally decompose at about 350xc2x0 C. Therefore, we can say magnesium hydroxide is the most preferable fire retardant for the heat-setting type epoxy resins. Additionally, if a little dehydration is allowed in the heat-setting process or if the thickness of the neutron shielding material is determined in advance to make up for the loss by dehydration, alumina trihydrate can be added to the high-temperature setting epoxy resins. We inventors also found that it is possible to add phosphor compound and halide type fire retardants (which have been widely used as fire retardants in general industrial fields) to heat-setting epoxy resins to be used for neutron shielding judging from the discussions similar to those on magnesium hydroxide. As explained above, we inventors can clarify a standard for selecting fire retardants available to heat-setting epoxy resins to be used for neutron shielding materials from our results of tests on fire retardants.
Neutron shielding materials made of heat-resistant high-polymer materials can contain a neutron absorbing material such as a boron compound which has a great neutron-absorbing sectional area. The typical example is the neutron shielding material used in a metal cask. For this purpose, boron carbide is added to room-temperature setting type epoxy resins. When heat-setting epoxy resins are applied as neutron shielding materials, it is peculiar that the neutron-absorbing component (e.g. boron carbide and boron nitride) may react with components of the heat-setting epoxy resin such as base resin, hardener such as acid anhydride, and fire retardant such as magnesium hydroxide at high temperature. Experimentally, we inventors have ascertained that boron carbide and boron nitride do not react with components of the heat-setting epoxy resin such as base resin, hardener such as acid anhydride, and fire retardant such as magnesium hydroxide at high temperature. From the above discussion, we inventors hit on an idea that we can improve the neutron shielding performance of the neutron shielding material made of a heat-setting epoxy resin without losing the characteristics of the hardened material by adding a boron compound such as boron carbide and boron nitride as the neutron absorbing material to the epoxy resin. For the similar reason, non-boron compounds such as gadolinium oxide and samarium oxide are also available as neutron absorbing materials.
As apprarent from the above explanation, the present invention has been made from our own profound experiments and evaluations which clarify that the heat-setting epoxy resins can be applied as neutron shielding materials.
The base resin is preferably a member selected from the group of bisphenol A epoxy compound, novolak epoxy compound, alicyclic glycidyl ether type epoxy compound, various glycidyl ester type epoxy compound, glycidyl amine type epoxy compound, and biphenol type epoxy compound or a mixture thereof.
The hardener is preferably a member selected from the group of amine type hardener such as aromatic amine, alicyclic amine, and polyamide amine, acid anhydrate type hardener, and imidazole type hardening promoter or a mixture thereof. Below will be explained the quantity of respective hardeners to be added to the base resin. If the equivalent ratio of epoxy groups in the base resin to active hydrogen atoms in the hardener is not 0.7 to 1.3, excessive base resin or hardener may be left in the hardened resin. As both the base resin and the hardener have vapor pressures, the excessive base resin or hardener will evaporate when heated up. This causes a loss of hydrogen atoms in the hardened resin. Therefore, the equivalent ratio of epoxy groups in the base resin to active hydrogen atoms in the hardener should be 0.7 to 1.3 and more preferably about 1.0 to make the hardened resin has a low weight reduction rate due to heat.
The fire retardant is preferably a member selected from the group consisting of metal hydroxide such as magnesium hydroxide, aluminum hydroxide, and calcium hydroxide, hydrates of said metal oxide, inorganic phosphoric compounds such as ammonium phosphate, organic phosphoric compounds such as phosphoric ester, and halogenated compounds such as hexabromo benzene and tetrabromobisphenol A or a mixture thereof. As the hardened resin contains more fire retardant, the hardened resin can be more fire-resistant but reversely the hardened resin has less hydrogen number density and becomes more viscous. To give a high shielding performance, fire resistance and workability to the hardened resin in a well-balanced manner, the ratio of the fire retardant should be 30% to 60% by weight.
The neutron absorbing materials are preferably isotopes having a high thermal neutron absorbing sectional area and more preferably boron compounds such as boron carbide and boron nitride, cadmium compounds such as cadmium oxide, gadolinium compounds such as gadolinium oxide, and samarium compounds such as samarium oxide. However in general, these compounds are very expensive, the minimum ratio of addition can be determined from the neutron absorbing performance and maximum ratio of addition can be determined from the cost of the neutron shielding material. From our cost and performance estimation, the preferable ratio of the neutron-absorbing material is 0.1% to 10% by weight to get a neutron shielding material having a balanced relationship of shielding cost and performance.
Preferale metal hydrides are titanium hydride and preferable hydrogen-absorbing alloys are magnesium-nickel alloy.
The neutron shielding material of the present invention is hard to lose the neutron shielding performance even when exposed to a high temperature of 150 to 200xc2x0 C. and has the neutron shielding ability improved in that temperature range.